Torque limiting device



y 1954 R. N. PRITTIE TORQUE LIMITING DEVICE Filed April 16. 1952 FIGINVENTOR. ROBERT N. PR/TTIE I A A T TOR/VE Y Patented May 25, 1 954TORQUE LIMITING DEVICE Robert N. Prittie, Chagrin Falls, Ohio, assignorto Jack & Heintz, Inc., Cleveland, Ohio, a corporation of DelawareApplication April 16, 1952, Serial No. 282,666

5 Claims.

This invention relates in general to improvements in torque limitindevices, and more particularly to those employing a multiple frictiondisc assembly.

The conventional torque limiter for planetary gear trains consists of apack of discs with steel internal gear discs assembled alternately withexternally splined bronze discs. The splined bronze discs mate with aninternal spline in the gear housing. With the pack spring-loaded aX-ially, the internal gear discs form the fixed memher, under normaloperating conditions, of a stage of planetary reduction gearing. Whenthe torque load on the planetary carrying arm increases sufficiently toovercome the friction between the spring-loaded discs, the carrying armstops and the internal gear discs slip.

Because the coefficient of starting friction is higher than thecoefficient of sliding friction, the torque load on the carrying armrequired to start the internal gear discs slipping is higher than thatrequired to keep the discs slipping. This ratio of breakaway torque toslipping torque becomes excessive under certain condi tions. This isespecially evident at low temperature, under which condition theshearing resistance of the lubricant used in the pack of discs increasesappreciably.

' In such torque limiting devices, that are used in connection withplanetary gear trains, dynamic breakaway torque is the maximum torqueexerted on the planetary gear carrying arm as the result of suddenlyarresting the rotation of the carrying arm, such as by engagement with afixed jaw clutch. This causes the internal gear to slip, while the inputtorque is still being applied to the sun gear of the planetary gearingstage. Dynamic breakaway torque is distinguished from static breakawaytorque as the latter is the torque exerted on the planetary carrying armwhen locked before input torque is applied to the sun gear, causing theinternal gear discs to slip.

One of the primary objects of my invention is to provide an improvedtorque limiting device that will inherently have a low ratio of dynamicbreakaway torque to slipping torque throughout a Wide temperature range.

With the foregoing and other objects in View, the invention resides inthe combination of parts and in the details of construction hereinafterset forth in the following specification and appended claims, certainembodiments thereof being illustrated in the accompanying drawings, inwhich:

' Figure 1 lea-fragmentary view in longitudinal 2 section, showing adriving shaft and a driven shaft with a planetary gear train andmultiple friction disc torque limiting device; and

Figure 2 is a detail enlarged view of my novel friction disc assembly.

Referring more particularly to the drawings, I have shown my noveltorque limiter multiple friction disc assembly as applied to aconventionalplanetary gear train, as may be found between the motordriving shaft and a driven output shaft.

Inside of housing i, a driving shaft 2 is mounted for rotation in a ballbearing 3'. Shaft 2 has keyed thereto at 4, near one end, a sun gear 5of a planetary reduction gear train. In Figure 1, one of the usualplanetary gears is shown at 5 meshing with sun gear 5 and rotatablysupported by a cross pin 7 in a planetary gear car rier 8 that issupported by ball bearings 9. The carrier 8 is internally splined tomate with external splines on an output shaft S0 to drive the latter.

A multiple friction disc brake assembly is generally indicated at i i inFigure 1. It consists generally of a series of centrally apertured discsencircling the planetary gear assembly and is disposed between astationary frame member l2 and a linearly slidable plate it that isurged to the right by compression springs Hi to compress the frictiondisc assembly i i.

This friction disc assembly is shown in greater detail in Figure 2. Forpurposes of illustration, there are shown three pairs of bronze discsindicated at l5, l6 and il, all of which are provided withcircumferential teeth to mate with internal splines on the housing I topermit linear movement but prevent rota-tive movement of these bronzediscs. Between these pairs of bronze discs are arranged two pairs ofsteel discs IS and i8 and i9 and i9 that are provided with a series ofteeth to mesh with the external teeth of the planetary gears t. Inbetween the pair l8 and i8 is arranged an untoothed bronze disc 20 andin between the pair i9 and in between the pair l 9 and is is arranged anuntoothed bronze spacer disc 2!.

It has long since been conceded that the use of lubricant between theopposed faces of the discs, as well as means for preventing its escape,is imperative for the proper operation of such a torque limiter. standhigh temperatures are inclined to materially increase in viscosity atlow temperatures. This results in the above mentioned high break-However, lubricants that can away torque when the torque limiting deviceis required to slip.

This is the reason for my departure, as shown in Figure 2, from theconventional. The bronze external gear discs, arranged in pairs l5, Itand I! are conventional and have a standard number of circumferentialteeth.

As to the steel internal gear discs, I provide one of each pair withless internal gear teeth than the other. As an example, I will refer toouter gear discs l3 and Is as having a standard number of 87 internalgear teeth and special inner gear discs l8 and is as having 90 internalgear teeth. The bronze spacers 2t and 2| are added to separate thestandard disc Hi from special disc l8 and standard disc 19 from specialdisc; Hi.

In operation, as soon as the motor shaft 2 begins to rotate, there willbe relative rotation between each standard internal gear disc iii and I9and its adjacent special internal gear disc l3 and I8, respectively.Thus, the standard and special internal gears, in bearing on theopposite faces of their common intermediate bronze spacer, are forced torotate with respect to each other, although at a low rate as comparedwith the speed of the output shaft Hi. This retation against thefrictional resistance produced by the springs i i generates heat so asto reduce the viscosity of the lubricant grease in the assembly. Thegrease is also sheared, which, to-- gether with the lowered viscositytends to reduce the dynamic breakaway torque of the unit.

All of. this occurs rapidly, inasmuch as the standard and specialinternal gears rotate relatively to each other at low torques, evenwhile the friction disc assembly, as a unit, remains stationary tofunction as a means for driving the output shaft.

slippage, or relative rotation, between the discs of the torque limitingdevice causes heat to be generated, which first reduces the viscosity ofthe lubricant and then causes the lubricant to be sheared oil at thedisc faces. t the instant that excessive torque is imposed upon thetorque limiting device, tl-e dynamic breakaway torque will beconsiderably less than that of a conventional torque limiting devicewith the same slip setting. This is because the lubricant will have beensheared during any operation prior to the instant that excessive torqueoccurs.

In my illustrative example with respect to standard and special internalgear discs, I have in mind the use of the planetary gears. I have named87 teeth as standard and 90 as special. This is merely arbitrary exampleand could be 87 and 8% or any other number with increments of 3 toothdifference. Thus, if two planets are used, the tooth differential shouldbe in increments of 2.

It is also to be understood that, Whereas I have described my inventionin connection with a torque limiter using lubricant, it also has utilityin a dry torque limiter.

I claim:

1. In combination with a planetary gear train mounted in a hOLlSll'lgand arranged between a driving input shaft and a driven output shaft,said gear train having an externally toothed pinion, a multiple frictiondisc torque limiting device having alternately arranged pairs ofstationary and rotatable discs normally spring pressed into frictionalengagement, said stationary discs bein peripherally toothed to engageteethonthe inner surface of said housing and said rotatable discs havinginternal teeth to mate with the external teeth of said pinion, one discof each pair of rotatable discs having a greater number of internalteeth than the other so as be rotated relatively to each other by saidplanetary gears, so as to generate frictional heat and reduce theviscosity of lubricant and shear the same in the device adjacent saidmultiple disc assembly.

27 In combination with a planetary gear train mounted in a housing andarranged between a driving input shaft and a driven output shaft, saidtrain having an externally toothed pinion, a multiple friction disctorque limiting device having alternately arranged pairs of stationaryrotatable discs normally spring pressed into frictional engagement, saidstationary discs being peripherally toothed to engage teeth on the innersurface of said housing and said discs having internal teeth to matewith the external teeth of said pinion, a spacer disc arranged betweeneach pair of rotatable discs, one disc of each pair of rotatable discshaving a greater number of internal teeth than the other so as to berotated relatively to each other by said planetary gears so as togenerate frictional heat and reduce the viscosity of lubricant and shearthe same in the device adjacent said multiple disc assembly.

3. In combination with a planetary gear train mounted in a housingarranged between a driving input shaft and a driven output shaft, saidgear train having an externally toothed pinion, a multiple friction disctorque limiting device havin alternately arranged pairs of relativelysoft stationary relatively hard rotatabl discs, normally spring pressedinto frictional engagement, said soft discs being peripherally toothedto engage teeth on the inner surface of said housing and said hard discshaving internal teeth to mate with the external teeth of said pinion,one disc of each pair of hard discs having a greater number of internalteeth than the other, so as to be rotated relatively to each other bysaid planetary gears so as to generate frictional heat and reduce theviscosity of lubricant and shear the same in the device adjacent saidmultiple disc assembly.

In combination with a planetary gear train mounted in a housingandarranged between a riving input shaft and a driven output shaft, saidgear train having an externally toothed pinion, a multiple disc frictiontorque limiting device having alternately arranged pairs of relativelysoft stationary, and relatively hard rotatable discs, normally springpressed into non rotative frictional engagement, said soft discs beingperipherally toothed to engage teeth on the inner surface of saidhousing and said hard discs having internal teeth to mate with theexternal teeth of said pinion, a relatively soft spacer disc arrangedbetween each pair of hard discs, one disc of each pair of hard discshaving a greater number of internal teeth than the other so as to bedriven relatively to each other by said planetary gears so as togenerate frictional heat and reduce the viscosity of lubricant and shearthe same in the device adjacent said multiple disc assembly.

5. In combination with a planetary gear train mounted in a housing andarranged between a drivin input shaft and a driven output shaft, saidgear train having an externally toothed pinion, a multiple friction disctorque limiting device having. alternately arranged pairs of sta-References Cited in the file of this patent UNITED STATES PATENTS NumberName Date Simpson Oct. 17, 1922 Heintz Feb. 3, 1942 Heintz Dec. 1, 1942Cahill Apr. 22, 1947 Dixon May 13, 1947 Howard Feb. 5, 1952

